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The app is pretty simple to use. Once installed, it adds keyboard shortcuts that quickly move and resize the active window to the top left/right, bottom left/right, left/right/bottom/top half, or center of your screen. Hit Opt+Cmd+F to go fullscreen (like Windows' maximize). You can refresh yourself on any of the shortcuts by clicking the Shift drop-down in the menu bar.

ShiftIt is a free download for Mac OS X only. Pre-Windows 7 users, check out previously mentioned AeroSnap to get similar functionality in Vista or XP.